A 5' LTR-cell DNA junction fragment from a MoMuLV induced rat thymoma has been cloned in bacteriophage lambda. Tumor cell DNA from 16 tumors has been examined for DNA rearrangements within the cellular sequences represented in this clone. We have shown that in 3 out of 16 tumours these cellular sequences have been rearranged due to virus integration. This is the second preferred virus integration event we have identified in these tumors (See Annual Reports by P. N. Tsichlis and L. F. Hu). In one tumor, both of the so far identified preferred integration events, appear to be present simultaneously. Analysis of virus related cDNA clones obtained from the same tumors indicates the generation of virus recombinants in this system.